Three-way catalysts for simultaneously purifying carbon monoxides (CO), hydrocarbons (HC) and nitrogen oxides (NOx) contained in the exhaust gas usually support noble metals, such as platinum, rhodium and palladium, and are now in wide use as exhaust gas purifying catalysts. Of these noble metals supported in the exhaust gas purifying catalysts, rhodium exhibits excellent activity for both oxidation and reduction and develops an excellent effect in purification of NOx in particular. For this reason, a variety of proposed exhaust gas purifying catalysts employ rhodium as their essential component and support platinum or palladium in combination with rhodium.
In the meantime, rhodium is expensive and there has been a strong industrial desire to produce an exhaust gas purifying catalyst that requires a smallest possible quantity of rhodium for effectively developing the equivalent effect.
Turbocharged engines and light load vehicles are low in exhaust gas temperature and, accordingly, it is necessary for those engines and vehicles to effectively develop the activity of the exhaust gas purifying catalyst even in low temperature.
It is an object of the present invention to provide an exhaust gas purifying catalyst that provides the excellent low-temperature activity and develops the good catalytic activity of rhodium to produce an excellent purifying performance economically.